Tuesday, 9 April 2013

Rubiaceae and some of its’ species on the uMngeni Estuary

Rubiaceae and some of its’ species on the uMngeni Estuary

There are two species along the River and they also grow along the KZN Coastal belt. These are Psychotria capensis Black Bird-berry and Pavetta lanceolata (Weeping Bride’s bush) They belong to this huge family Rubiaceae that goes back to the Eocene, which was 34 to 56 million years ago. The dinosaurs had just become extinct. During the Eocene, greenhouse gases (carbon dioxide and methane) played an important role in controlling surface temperatures. The diversity for this family is greater in the humid tropics.  Coffea (coffee) belongs to this family as does Chinchona (quinine).
Pavetta lanceolata is a shrubby plant that grows from about 2-4m. It grows in full sun. It has beautiful masses of scented white flowers, hence it’s common name, like confetti. These occur in mid-summer.
 Pavetta might come from the word pauvatta, a Sinhalese (Sri Lanka) vernacular word for this genus. There is also another interesting possible meaning. Pavimentum is Latin for pavement/bricks/stones. Therefore a mosaic. This could refer to the scattered bacterial nodules that are visible on the leaves. 
 Psychotria capensis grows in half-sun. It gets to about 2-3m high. Looks better growing in a clump with a tree, or other bushes. The flowers are little yellow ones that come from August to January and the berries (from yellow to black to red) from January to July. The black-eyed bulbuls (topies) adore them.  This word comes from the Greek and it means vivifying, to give or endow life. In the Amazon, there are many species that are used by the shamans for a variety of purposes.
 Along the river, the bacterial nodules are more visible on Pavetta lanceolata. Other Psychotria species have more scattered nodules. Bacterial nodules are small swellings which houses symbiotic bacteria, which fix atmospheric nitrogen, into a form that the plants can use. There are about 500 flowering species in the Rubiaceae worldwide, that have these endosymbionts. These symbionts are known to play a role in assisting in survival of the host. The loss of this bacterial partner/endophyte affects growth and development of the host plant, suggesting an altered phytohormonal balance.
The ancestors of this leaf -nodulated bacteria-plant association originated probably about 13 Million years ago, during the Miocene (5-23 Million years). 15 Million years ago, the land mass, which is known today, as Antarctica, split from Gondwana. It went to the South Pole. In doing so, it left behind the warm currents of the tropics and the climatic conditions changed so much, that massive ice-sheets began to be formed. This also happened in the Arctic. Aridification started in Africa and Asia. The sea-levels and temperatures, around the world started to drop.  As a result of a drier climate, many plants died out, or had to adapt. It is possible to assume that climatic change was a possible trigger to promote the bacteria-plant associations, among leaf-nodulated species, in Africa and Asia. The nodules might have become a safe haven for soil bacteria that were confronted with drier habitats. The plant, in turn, had enhanced growth, while it was under drought-stress. As Antarctica started to melt, around 5 million years ago, so the sea-levels rose.

Boon, R. 2010. Pooley’s Trees of Eastern South Africa. A Complete Guide. Flora &Fauna Publications Trust, Durban
Glen, H. 2004. Sappi What’s in a Name? Jacana, Johannesburg
Nichols, G. 2002. Down to Earth: Gardening with Indigenous Shrubs. Struik
Norman, N & Whitfield, G. 2006. Geological Journeys. Struik. Cape Town

Photograph Notes:
The Pavetta and Psychotria flowers from http:// site
The Pavetta leaves taken by Kerry-lynn Rieckhoff
Endophyte – an organism that lives within another, without causing disease
Phyto – plants
Symbiotic – a close, prolonged association between 2 or more different organisms of different species that may but does not necessarily benefit each one

by Rosemary Harrison

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